1. Field of the Invention
This invention relates to optical rotation rate sensors and more particularly to ring laser rotation rate sensors.
2. Background Art
Optical rotation sensors such as gyroscopes are highly sensitive devices which, by the application of a relativistic effect commonly known as the Sagnac effect, are used to determine rotation of a vehicle about an axes. Typically, three separate gyros are used in aircraft and space vehicles to measure rotation about three orthogonal axes. Navigation and flight control information is derived from rotation measurements about the three orthogonal axes.
One type of optical rotation sensor, the so called fiber optic gyro (FOG), employs a coil of optical fiber through which light waves are transmitted in opposite directions. Optical waves emerging from the coil are combined and transmitted to a photodetector. Analysis, typically by means of a connected signal processor, of the interference pattern of the combined light waves at the photodetector provides an indication of rate and direction of rotation.
Another type of optical rotation sensor is the so called ring laser gyro (RLG). Typically, prior art RLG consist of a dimensionally-stable polygon with at least three edges with mirrors positioned at the corners. A cavity formed in the polygon contains a gas mixture which supports and amplifies counterpropagating laser beams. Counterpropogating laser beams outside the cavity create a standing wave pattern with high and low intensity nodes spaced apart by a distance equal to one-half the wavelength of the laser beam. As the gyro is rotated, the optical path length changes, as defined by the Sagnac effect. The change in path length is accompanied by a differential change in wavelength of the counter rotating beams and causes a shift in the position of the light intensity nodes. A photodetector directed at the position of one of the nodes detects an intensity change resulting from the change in wavelength when the gyro is rotated about its axis. Signal processing circuitry analyzes the intensity change detected by the photodetector and derives navigation and flight control information related to the rotation of the gyro from the change in light intensity.
Prior art ring laser gyros using a gas filled cavity and a mirror structure have the disadvantage that a proper gas mixture must be maintained in a confinement vessel and that high quality mirror surfaces must be fabricated and precisely configured and aligned in order to obtain a desired effect. Furthermore, a mechanical dithering mechanism is typically provided, further complicating the structure. It is therefore desirable to provide a ring laser gyroscope which does not use a gas and is less expensive to manufacture and smaller in mass and size dimensions, particularly for spacecraft applications, and which avoids the alignment and other mechanical problems of the prior art devices.
Experiments with integrated optic devices of various kinds have been performed including the implementation of rare earth doped waveguide lasers in both glass and lithium niobate. Fabrication of erbium doped waveguides and neodymium doped waveguide Fabry-Perot lasers in lithium niobate have been reported. Erbium and neodymium doped silica fiber lasers in both Fabry-Perot and ring-resonator configurations have also been reported. In separate developments, monolithic integration of ring-resonator waveguides with semiconductor lasers has been used in attempts to eliminate the requirements for cleaving or for distributed feedback structures in integrated photonic circuits. Ring-resonator lasers have also been produced using GaAs/GaAlAs and InGaAsP materials. However, no practical application of these various activities has been found which solve the problems inherent in prior art ring laser rotation sensors.
A particular problem in the design of optical ring rotation sensors is the need to produce counterpropagating optical waves required to sense the Sagnac effect that provides an indication of rotation. Bidirectional, counterpropagating optical waves may be produced in a gas ring laser. However, bidirectional waves produced in solid medium ring lasers tend to be unstable. The stable regimes in solid medium lasers exhibit lasing only in a single direction. This bidirectional instability is a consequence a phenomenon known as spatial hole burning in the gain medium, caused by the counterpropagating waves. In addition, backscatter between counterpropagating optical waves is known to cause frequency locking of the counterpropagating waves at low rotation rates, which prevents rotation rate sensing at the low rates. Prior art systems use complicated mechanical dither mechanisms to allow sensing at low rates.